As is known in the art, radar systems have been developed for various applications associated with vehicles, such as automobiles and boats. A radar system mounted on a vehicle detects the presence of objects including other vehicles in proximity to the vehicle. In an automotive application, such a radar system can be used in conjunction with the braking system to provide active collision avoidance or in conjunction with the automobile cruise control system to provide intelligent speed and traffic spacing control. In a further automotive application, the radar system provides a passive indication of obstacles to the driver on a display.
A continuing safety concern in the operation of automobiles is the difficulty in seeing objects in the side blind spots of the automobile. Accidents often occur when an automobile impacts another vehicle in its blind spot when changing lanes.
Rear and side view mirrors of various sizes and features are typically used in an effort to improve visualization of blind spots. For example, convex mirrors provide a larger view than flat mirrors. However, objects viewed in a convex mirror appear farther away than their actual distance from the vehicle. Also, the view through mirrors degrades during conditions of rain, snow, or darkness.
There is a need for an effective way to detect obstacles in a vehicle's blind spots, and generally in close proximity to the vehicle, which is accurate and reliable during all types of environmental conditions including rain, snow, and darkness. A further characteristic of an effective detection system is a well-defined detection zone within which there is a very high probability of detection, and outside of which there is a very low probability of detection.
As is known in the art, there are many types of radar transmission techniques, one of which is Frequency Modulated Continuous Wave (FMCW) transmission, in which the frequency of the transmitted signal increases linearly from a first predetermined frequency to a second predetermined frequency. FMCW radar has the advantages of high sensitivity, relatively low transmitter power and good range resolution.
Various circuitry and techniques can be used to generate an FMCW transmit signal. One technique is to feed a signal voltage having a ramp characteristic (referred to herein as a “ramp signal” or “ramp voltage”) to a voltage controlled oscillator (VCO) to generate the frequency modulated transmit signal commonly referred to as a chirp signal. Typically, the ramp signal is generated by an analog circuit that may include timing pulse generation circuits, integrators and amplifiers. Components of such an analog circuit are fixed at the design stage and thus, such a circuit does not afford much, if any versatility.
Ideally, the frequency of the VCO output signal varies linearly with respect to the ramp voltage. When there is non-linearity in the ramp signal and/or in the operation of the VCO, the frequency of the RF return signal can be spread across an RF frequency range or “smeared”, thereby degrading target detection, resolution and range accuracy performance of the radar system.
Another technique for generating an FMCW transmit signal is to use direct-digital synthesis (DDS) in which the transmit signal itself is digitally synthesized. Typical DDS systems include a phase accumulator and a digital-to-analog (D/A) converter. However, the transmit signal rate is limited by the Nyquist theory to less than one-half of the maximum clock rate of the D/A converter. Other disadvantages of DDS systems include complexity and cost plus an increase in supporting hardware requirements because of limitations in operating frequency and tuning range of currently available DDS synthesizers.
As is also known, some relatively complex radar systems include multiple transmit and receive circuits (TRCs) each of which operate independently of one another. When such transmit and receive circuits are placed in proximity to one another and operate at the same or overlapping frequencies, the multiple TRCs can interfere with one another, preventing the accurate detection of targets. Other problems can also arise by simultaneous operation of multiple TRCs.
Radar systems provide several design challenges. As one example, when radar systems operating at the same or overlapping frequencies are used in proximity to one another, the two systems can interfere with one another, preventing the accurate detection of targets. For example, circuit performance variations attributable to temperature changes can result in interference between multiple TRCs. It would, therefore, be desirable to provide a radar transmitter circuit which permits adjustment of transmit signal characteristics in a relatively simple manner. It would also be desirable to provide a radar transmitter which compensates for variations in transit signal characteristics caused by variations in temperature in the environment in which the radar transmitter is disposed. It would be still further desirable to provide an FMCW radar system which compensates for non-linear VCO operation. It would be still further desirable to provide a technique which allows simultaneous operation of multiple TRCs in overlapping frequency ranges. It would be still further desirable to provide a system and technique which allows simultaneous operation of multiple FMCW TRCs that provides for changing radar coverage.